DirectX is a collection of APIs for Windows that offer a useful layer of abstraction between software and hardware. Everything that does digital media on Windows — including streaming platforms like QuickTime and RealSystem — interacts with DirectX in order to talk to your graphics and sound hardware.
DirectX 9.0’s main benefits are that (1) it can theoretically improve the stability, performance and visual quality of games if you have 3D hardware which has DirectX 9.0-enhanced drivers (ATI is the first with its Catalyst 3.0 drivers), (2) it apparently fixes lots of problems with DirectPlay, and (3) it enhances DirectPlay by using with UPnP (universal plug and play) routers to open and close TCP/IP ports needed for multiplayer gaming as necessary. (It also finally incorporates that pesky Video Capture Update for DirectX 8.0 that you had to remember to install on any machine you wanted to capture video with.)
However, not all is sunshine and roses. For example, DirectX 9.0 breaks multiplayer gaming on Microsoft’s own Flight Simulator 2002.
This release also includes Microsoft’s High Level Shader Language (HLSL). Although Nvidia’s “our stuff works fine with DirectX 9.0″ press release includes a quote from Microsoft saying that Nvidia’s Cg shader language is A Good Thing, HLSL is effectively a competitor to Cg and will kill it by the end of 2003. This means that there’s no longer anything differentiating Nvidia from ATI except performance, and on that front it’s questinable whether the once-seemingly-unbeatable Nvidia can compete with a very hungry ATI. | Download DirectX 9 | ATI press release